Calculator guide
Period Calculator helps you explore the stated health question. Start with the labelled inputs, then review the method, worked example and limitations before using a result.
Formula or method
Expected next period = last period start + entered average cycle length. Additional dates repeat that assumed cycle length.
How to use this calculator
- Enter the last period start date and usual cycle length.
- Review estimated dates as a range of possibilities; cycles need not follow an exact schedule.
Worked example
With a period starting January 1, 2026 and a 28-day cycle, the next estimated start is January 29, 2026.
Understanding the result
Compare the output with the period calculator worked example using exactly the same inputs. A different assumption or rounding convention can produce a different displayed result; the output does not establish personal or professional suitability.
Limitations and assumptions
- Actual cycles can vary. Calendar estimates are not reliable contraception and cannot determine whether pregnancy is present.
- The worked example demonstrates the stated assumptions, not a recommendation about your personal circumstances.
Frequently asked questions
Which inputs should I check first?
Follow the Period Calculator input instructions above. Check each measurement, date, selection or expression before comparing results.
Why might my result differ from another tool?
Compare the other tool's period calculator method, input units, rounding and assumptions. A matching title does not guarantee that both tools answer the same question.
Does a linked reference certify this result?
No. A reference can explain a formula or provide background for Period Calculator, but it does not endorse this website, validate personal inputs or establish professional suitability.
Sources
Educational and planning information only. Sources are attributed background or formula references, not endorsements. Verify consequential decisions with an appropriate qualified professional.